Probably the most familiar vertebrate animals around the world, occupying every continent and every habitat type. Birds are warm blooded (endothermic), have feathers and lay eggs that are protected by a hard outer shell.
Most scientists agree that the evidence suggests that birds evolved from small non-flying theropod dinosaurs. The first of these avian-dinosaurs to be discovered was the Archaeopteryx lithographica, dated at around 150 million years, which was first described by science in 1860, although the origins of birds date before then. A more recent fossil bird from 125 million years ago (mya), discovered in China, Confuciusornis sanctus, displayed more of the characteristics of the modern birds, such as a toothless beak, but both fossils had feathers. Many non-avian dinosaurs, the deinonychosaurs, shared a common ancestor, and also displayed many bird-like characters, including primitive feathers on their forelimbs, tail and body, and hollow, thin-walled bones. Modern (toothless) birds then evolved in the late Cretaceous period, around 100 mya.
All modern birds are housed in the subclass Neornithes, which branch into two main groups: the Palaeognathae, or palaeognaths, which comprises the ratites (ostrich, emus, cassowaries, rheas and kiwis, as well as the recently extinct moas and elephantbirds) and tinamous, and the Neognathae, which is made up of all the other birds. This then divides into the Galloanseres (waterfowl [Anseriformes] and landfowl [Galliformes]) and the Neoaves (all others). The Neoaves are then split further by some authorities into the Metaves (Australian representatives are the tropicbirds, grebes, flamingoes, pigeons, frogmouths, nightjars, owlet-nightjars and swifts) and the Coronaves (all other families).
In the past twenty years or so, the relationships within birds has undergone much molecular and morphological study, and a great degree of turbulence in classification and nomenclature has been seen as a result. Although general agreement is being achieved in many areas of avian phylogeny, studies are ongoing, and disagreement and debate will inevitably continue between scientists as a result of the various findings and recommendations.